1. Field of the Invention
The present invention relates to novel electrolyte compositions for high energy density electrochemical cells and to cells including these electrolyte compositions. More specifically, the present invention is directed to electrolyte compositions consisting essentially of solvent and electrolytically active alkali metal salts including an organometallic alkali metal salt. It is also directed to rechargeable, high energy density electrochemical cells having alkali metal anodes and containing these electrolyte compositions.
2. Prior Art
A recently developed rechargeable, high energy density electrochemical cell consists of an alkali metal material as the anode-active material, a transition metal chalcogenide as the cathode-active material, and a nonaqueous electrolyte. More specifically, preferred cells consist of lithium anodes, titanium disulfide cathodes and nonaqueous electrolyte compositions consisting of various lithium salts, such as LiClO.sub.4, dissolved in organic solvents, such as propylene carbonate, tetrahydrofuran, dioxolane, and mixtures of dimethyoxyethane and tetrahydrofuran, and containing various stabilizing additives.
Important features of these cells include their ability to be repeatedly discharged and charged. Theoretically, cycling by discharging and charging should be possible indefinitely, but in practice indefinite cycling is not realized. Dendritic growth on the anode during charging and degradation of the cathode material are sometimes limiting factors in the amount of cycling to which a cell can be subjected. However, the electrolyte, particularly nonaqueous electrolytes, can at times be the limiting factor. The effects of a particular electrolyte composition on the electrochemical performance of a cell may be significant due to its relative stability or it may be due to other factors. One particular electrolyte composition might be highly effective with a given anode-cathode couple but be ineffective for another couple, either because it is not inert to the second couple or because it reacts with itself under the conditions present during cycling. Furthermore, even when a particular electrolyte composition is effective in a given cell, it may nonetheless be undesirable for other reasons. For example, the sometimes preferred LiClO.sub.4 based electrolyte creates a potential explosion hazard. And, for example, various organometallic alkali metal salt compounds such as are described in U.S. Pat. Nos. 3,734,963 and 3,764,385 have the disadvantage of requiring complexing with various nitrogen, phosphorus or sulfur-containing organic compounds containing at least two functionalities. Recent studies have been made directed to LiB(C.sub.6 H.sub.5).sub.4 electrolyte systems by Szwarc et al, J. Phys. Chem., Vol. 69, p 608 et seq. (1965) but these systems have been found to have low solubility and high resistivity. For these reasons, novel but effective lithium salt-containing electrolyte compositions for alkali metal anode type cells are desirable.